Two major issues for EUV lithography (EUVL) manufacture are low source power and defective pellicles. A pellicle is employed in deep ultraviolet lithography (DUVL) to block particles from the mask pattern surface. The pellicle is placed several millimeters (mm) from the pattern plane, such that particles on the pellicle are out of focus, and, therefore, cannot be imaged. An EUV pellicle must have EUV transmission of at least 90% for a single pass and 81% for a double pass, and must have an open area of at least 110 by 142 mm2. However, since most materials absorb EUV wavelengths, building an effective pellicle for an EUV mask is difficult. One attempt to build a pellicle involved etching a membrane onto a wafer. However, the resulting pellicle, a film having a diameter of 80 mm and a film thickness of 25 nm, only achieved 86% single pass transmission. Further, the membrane was very fragile, which increases cost.
Even if a pellicle were formed with the proper specifications, mounting the pellicle onto an EUV mask also poses problems. For example, as illustrated in FIG. 1, a membrane 101 and silicon frame 103 are etched on a wafer (not shown for illustrative convenience) and then transferred to EUV mask 105 having a pattern area 107, such that the mask, membrane (the pellicle), and frame form a closed or semi-closed area. The membrane is thin, having a thickness 109 of tens of nanometers (nm), and has an area of 110 by 142 mm, and the silicon frame has a height 111 of at least several mm. If the air flow between the inside of the closed area and outside the closed area is not smooth and the air pressure difference on the two sides of the membrane is large, the thin membrane may be damaged. The mask is stored in normal air pressure, but when it is mounted into the EUV scanner, it must be vacuumed. Further, when the mask is shipped by air, there will also be air pressure changes.
A need therefore exists for methodology enabling fabrication of an EUV mask having a pellicle mounted thereto with sufficient air flow between the space enclosed between the mask and the pellicle and the space outside the mask and pellicle and for the resulting mask. Another need is to directly fabricate pellicle onto the EUV mask so the pellicle is not damaged during mounting on the EUV mask.